A mid‐Proterozoic coupled Sr and Li isotope excursion and its potential connection to enhanced weathering and ocean oxygenation at 1.57 Ga

• The first coupled positive seawater 87 Sr/ 86 Sr (∼0.0007) and δ 7 Li (∼5‰) isotope excursions identified in the Mesoproterozoic Era • Sr–Li isotopes and box modelling indicate enhanced silicate weathering and reduced congruency at 1.57 Ga • Weathering pulse coincides with ocean oxygenation and earliest decimetre-scale eukaryotes Chemical weathering is a critical Earth system process that regulates climate, ocean chemistry and the long-term carbon cycle. However, the intensity and variability of chemical weathering remain insufficiently constrained for the mid-Proterozoic (∼1.8‒0.8 Ga), greatly limiting our understanding of the environmental context to early eukaryotic evolution. Here, we report the first coupled positive seawater 87 Sr/ 86 Sr (∼0.0007) and δ 7 Li (∼5‰) isotope excursions of the Mesoproterozoic Era (1.6‒1.0 Ga), which we argue signifies a substantial weathering event at ∼1.57 Ga, characterised by increased silicate weathering rates and decreased weathering congruency. Drawing on independent geological evidence, we posit that enhanced volcanic CO 2 degassing, possibly alongside accretional orogenesis, increased denudation rates and invigorated the hydrological cycle, amplifying silicate weathering and secondary clay formation. This weathering pulse broadly coincided with coeval ocean oxygenation and carbon cycle disruption, implicating it in the appearance of the earliest known decimetre-scale, multicellular eukaryotic fossils.